Conventional cast iron is a ferrous alloy containing carbon. Cast irons are classified according to the shape of the carbon in the iron, also known as graphite morphology. The precipitated graphite in grey cast iron, the first developed and widely used cast iron, is in the shape of flakes. Grey cast iron has, however, some disadvantages such as a low tensile strength and low ductility.
Ductile iron, otherwise known as nodular iron, has a higher strength and ductility than normal grey cast iron. A spheroidizing agent, typically magnesium, cerium, or a combination of magnesium and cerium, is added to the iron which causes the precipitated graphite to form into a spherical shape instead of the irregularly shaped flakes of grey iron. These spheres, or nodules, give ductile iron its increased strength and ductility versus normal grey iron.
Ductile iron is classified into different grades based on the mechanical properties of the iron, such as tensile strength, yield strength, percent elongation, and hardness of the iron. The mechanical properties of ductile iron may be varied by controlling the matrix structure of the iron. For example, normal as-cast ductile iron consists of graphite nodules in a matrix of ferrite and pearlite, with a small amount of carbide as an undesirable constituent. Conventionally, the presence of carbides has been considered to be detrimental to as-cast ductile iron, and accordingly, as-cast ductile iron traditionally is produced with a limited amount of carbides. It is common for the maximum amount of carbide in as-cast ductile iron to be as low as 3%. Carbides have traditionally been disfavored in as-cast ductile irons because it was believed that they make the iron brittle.
Heat treatment has traditionally been used to change the matrix structure of the iron. Conventional heat treatments include normalizing and tempering, oil quenching and tempering, and austempering. Austempering has increasingly become a popular form of heat treating ductile iron. Austempering consists usually of heating the iron casting to approximately 1600-1700° F. and then holding the iron casting for sufficient time to allow the microstructure to homogenize. After the holding period, the casting is submerged and held in a medium at a lower, but still elevated temperature of 400-750° F. After the second holding period, the casting is cooled to room temperature. The austempering heat treatment transforms the microstructure of the ductile iron and reduces the carbide content. After the austempering treatment, the microstructure of the austempered ductile iron consists of graphite nodules in a matrix of ausferrite. Carbidic austempered ductile iron, used in high wear applications, contains more carbides than normal austempered ductile iron and has a matrix structure of ausferrite, high carbon retained austenite and 10-40% carbides.
While austempering increases the strength of the iron, it also adds increased time and expense to the casting process. Many iron applications require high wear resistance but do not necessarily require the increased strength provided by austempering. Thus, a need in the art exists for an iron with adequate wear and toughness properties which is more time and cost effective than austempered iron.
A general object of the present invention is the provision of an as-cast carbidic ductile iron manufactured without an austempering step.
A further object of the present invention is the provision of an as-cast carbidic ductile iron which has high abrasion wear resistance.
A still further object of the present invention is the provision of an as-cast carbidic ductile iron which has high sliding wear resistance.
A still further object of the present invention is the provision of an as-cast carbidic ductile iron which has a high toughness property.
A still further object of the present invention is the provision of an as-cast carbidic ductile iron which has a high hardness property.
A still further object of the present invention is the provision of an as-cast carbidic ductile iron which adequately balances strength, toughness, and wear resistance properties.
A still further object of the present invention is the provision of an as-cast carbidic ductile iron which provides high abrasion and sliding wear resistance properties at a lower cost than alternative materials.
A still further object of the present invention is the provision of an as-cast carbidic ductile iron which provides high toughness and hardness properties at a lower cost than alternative materials.
A still further object of the present invention is the provision of an as-cast ductile iron which provides high abrasion and sliding wear resistance properties and which requires less time to manufacture than alternative materials.
A still further object of the present invention is the provision of a method for making an as-cast ductile iron with a higher percentage of carbides than prior as-cast ductile irons.
A still further object of the present invention is the provision of a method for making a ductile iron with high abrasion, sliding wear resistance, hardness, and toughness properties which does not require an austempering step.
A still further object of the present invention is an object manufactured from as-cast carbidic ductile iron.
A still further object of the present invention is a plow point manufactured from as-cast carbidic ductile iron.
These as well as other objects, features and advantages of the present invention will become apparent from the following specification and claims.